As an effective means for replacing conventional radiography known is a recording and reproducing method of radiation images using stimulable phosphors described in Japanese Patent O.P.I. Publication No. 55-12148. In the method, a radiographic image conversion panel (hereinafter, also simply denoted as panel) comprising a stimulable phosphor is employed, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation having passed through an object or having been radiated from an object, sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as “stimulating rays”) to release the radiation energy stored in the phosphor as light emission (stimulated emission), photo-electrically detecting the emitted light to obtain electrical signals, and reproducing the radiation image of the object as a visible image from the electrical signals. The panel, having been read out, is then subjected to image-erasing and prepared for the next photographing cycle. Thus, the radiographic image conversion panel can be used repeatedly.
In the radiation image recording and reproducing methods described above, a radiation image is advantageously obtained with a sufficient amount of information by applying radiation to an object at a considerably smaller dose, as compared to conventional radiography employing a combination of a radiographic film and a radiographic intensifying screen. Further, in conventional radiography, the radiographic film is consumed for every photographing; on the other hand, in this radiation image converting method, in which the radiographic image conversion panel is employed repeatedly, is also advantageous in terms of conservation of resources and overall economic efficiency.
The radiation image conversion panel employed in the radiation image recording and reproducing method basically comprises a support and provided thereon, a phosphor layer (stimulable phosphor layer), provided that, in cases where the phosphor layer is self-supporting, the support is not necessarily required. The stimulable phosphor layer comprises a stimulable phosphor dispersed in a binder. There is also known a stimulable phosphor layer, which is formed by vacuum evaporation or a sintering process, free from a binder, and which comprises an aggregated stimulable phosphor. There is further known a radiation image conversion panel in which a polymeric material is contained in the openings among the aggregated stimulable phosphor. On the surface of the stimulable phosphor layer (i.e., the surface which is not in contact with the support) is conventionally provided a protective layer comprising a polymeric film or an evaporated inorganic membrane to protect the phosphor layer from chemical deterioration and physical shock.
The stimulable phosphor, after being exposed to radiation, produces stimulated emission upon exposure to the stimulating ray. In practical use, phosphors are employed, which exhibit an emission within a wavelength region of 300 to 500 nm stimulated by stimulating light of wavelengths of 400 to 900 nm. Examples of such stimulable phosphors include rare earth activated alkaline earth metal fluorohalide phosphors described in Japanese Patent O.P.I. Publication Nos. 55-12145, 55-160078, 56-74175, 56-116777, 57-23673, 57-23675, 58-206678, 59-27289, 59-27980, 59-56479 and 59-56480; divalent europium activated alkaline earth metal fluorohalide phosphors described in Japanese Patent O.P.I. Publication Nos. 59-75200, 6-84381, 60-106752, 60-166379, 60-221483, 60-228592, 60-228593, 61-23679, 61-120882, 61-120883, 61-120885, 61-235486 and 61-235487; rare earth element activated oxyhalide phosphors described in Japanese Patent O.P.I. Publication No. 59-12144; cerium activated trivalent metal oxyhalide phosphors described in Japanese Patent O.P.I. Publication No. 55-69281; bismuth activated alkaline metal halide phosphors described in Japanese Patent O.P.I. Publication No. 60-70484; divalent europium activated alkaline earth metal halophosphate phosphors described in Japanese Patent O.P.I. Publication Nos. 60-141783 and 60-157100; divalent europium activated alkaline earth metal haloborate phosphors described in Japanese Patent O.P.I. Publication No. 60-157099; divalent europium activated alkaline earth metal hydrogenated halide phosphors described in Japanese Patent O.P.I. Publication No. 60-217354; cerium activated rare earth complex halide phosphors described in Japanese Patent O.P.I. Publication Nos. 61-21173 and 61-21182; cerium activated rare earth halophosphate phosphors described in Japanese Patent O.P.I. Publication No. 61-40390; divalent europium activated cesium rubidium halide phosphors described in Japanese Patent O.P.I. Publication No. 60-78151; divalent europium activated cerium halide rubidium phosphors described in Japanese Patent O.P.I. Publication No. 60-78151; divalent europium activated composite halide phosphors described in Japanese Patent O.P.I. Publication No. 60-78153. Specifically, iodide-containing divalent europium activated alkaline earth metal fluorohalide phosphors, iodide containing rare earth metal activated oxyhalide phosphors and iodide containing bismuth activated alkaline earth metal halide phosphors exhibited stimulated emission of high luminance.
Along with the spread of radiation image conversion panels employing stimulable phosphors is further desired an enhancement of radiation image quality, such as enhancement in sharpness and graininess.
The foregoing preparation methods of stimulable phosphors are called a solid phase process or calcination method, in which pulverization after calcination is indispensable, however, there were problems such that it was difficult to control the particle form affecting sensitivity and image performance. Of means for enhancing image quality of radiation images is valid preparation of fine particles of a stimulable phosphor and enhancing particle size uniformity of the fine stimulable phosphor particles, i.e., narrowing the particle size distribution.
Preparation of stimulable phosphors in the liquid phase described in Japanese Patent O.P.I. Publication Nos. 7-233369 and 9-291278 is a method of obtaining a stimulable phosphor precursor in the form of fine particles by adjusting the concentration of a phosphor raw material solution, which is valid as a method of preparing stimulable phosphor powder having a narrow particle size distribution.
Of rare earth activated alkaline earth metal fluorohalide stimulable phosphors, a phosphor having higher iodide content is preferred in terms of reduction of radiation exposure. This is due to the fact that iodine exhibits a higher X-ray absorption than bromine.
Radiation image conversion panels are excellent in view of the resource conservation and economic efficiency, since they can be repeatedly operated, but it is known that stimulable phosphors in the radiation image conversion panels tend to be damaged when the radiation image conversion panels are repeatedly exposed to X-ray for a long duration.
This so-called X-ray damage is caused by luminance degradation generated via damage of the stimulable phosphor substance itself affecting the stimulated luminescence intensity. Therefore, radiation image conversion panels exhibiting reduced X-ray damage (luminance degradation) together with improved stimulated luminescence, erasing characteristics and graininess are demanded specifically for radiation image conversion panels repeatedly operated for a long duration.
(Patent Document 1) Japanese Patent O.P.I. 2001-11440